FIGS. 1 and 2 show the structure of a counter-rotating axial flow fan according to the related art disclosed in Japanese Patent No. 4128194 (FIGS. 1 and 2). FIGS. 1A, 1B, 1C, and 1D are a perspective view as viewed from a suction side, a perspective view as viewed from a discharge side, a front view as viewed from the suction side, and a rear view as viewed from the discharge side, respectively, of the counter-rotating axial flow fan according to the related art disclosed in Japanese Patent No. 4128194. FIG. 2A is a vertical cross-sectional view of the counter-rotating axial flow fan of FIG. 1. FIG. 2B shows front blades of the counter-rotating axial flow fan of FIG. 1. FIG. 2C shows rear blades of the counter-rotating axial flow fan of FIG. 1. In FIG. 2, some reference numerals and dimensions are changed from those of Japanese Patent No. 4128194 for illustration. The counter-rotating axial flow fan according to the related art is formed by assembling a first axial flow fan unit 1 and a second axial flow fan unit 3 via a coupling structure. The first axial flow fan unit 1 includes a first case 5, and a first impeller (front impeller) 7, a first motor 25, and three webs 21 disposed in the first case 5. The webs 21 are arranged at intervals of 120° in the circumferential direction. The first case 5 has an annular flange 9 on the suction side at one axial end of the first case 5 in a direction in which axis A extends (in the axial direction), and an annular flange 11 on the discharge side at the other axial end of the first case 5. The first case 5 also has a cylindrical portion 13 between the flanges 9 and 11. The internal spaces of the flange 9, the flange 11, and the cylindrical portion 13 form an air channel. The flange 11 on the discharge side has a circular discharge port 17 formed therein. The three webs 21 are combined with three webs 45 of the second axial flow fan unit 3 to form three stationary blades 61. The first motor 25 rotates the first impeller 7 in the first case 5 in the counterclockwise direction as shown in FIGS. 1A and 1C (in the direction of the arrow R1 in the drawings, which will be referred to as “one direction R1”). The first motor 25 rotates the first impeller 7 at a rotational speed higher than the rotational speed of a second impeller (rear impeller) 35. The first impeller 7 has an annular member (hub) 27 fitted with a cup-shaped member of a rotor (not shown) fixed to a rotary shaft (not shown) of the first motor 25, and N (five) front blades 28 integrally provided on an outer peripheral surface of an annular peripheral wall 27a of the annular member 27.
The second axial flow fan unit 3 includes a second case 33, and a second impeller (rear impeller) 35, a second motor 49, and three webs 45 disposed in the second case 33 and shown in FIG. 2. As shown in FIG. 1, the second case 33 has a flange 37 on the suction side at one axial end of the second case 33 in a direction in which axis A extends (in the axial direction), and a flange 39 on the discharge side at the other axial end of the second case 33. The second case 33 also has a cylindrical portion 41 between the flanges 37 and 39. The internal spaces of the flange 37, the flange 39, and the cylindrical portion 41 form an air channel. The first case 5 and the second case 33 form a casing. The flange 37 on the suction side has a circular suction port 42 formed therein. The second motor 49 rotates the second impeller 35 in the second case 33 in the counterclockwise direction as shown in FIGS. 1B and 1D [in the direction of the arrow R2 in the drawings, which will be referred to as “the other direction R2”, that is, in the direction opposite to the rotational direction of the first impeller 7 (the direction of the arrow R1)]. As discussed earlier, the second impeller 35 is rotated at a rotational speed lower than the rotational speed of the first impeller 7. The second impeller 35 has an annular member (hub) 50 fitted with a cup-shaped member of a rotor (not shown) fixed to a rotary shaft (not shown) of the second motor 49, and P (four) rear blades 51 integrally provided on an outer peripheral surface of an annular peripheral wall 50a of the annular member 50.
As shown in FIG. 25, the front blades 28 are each formed of a swept-back blade. The front blades 28 each have a curved shape in which a recessed portion opens in the one direction R1 (the rotational direction of the impeller 7) discussed above as viewed in lateral cross section. As shown in FIG. 2C, the rear blades 51 are also each formed of a swept-back blade. The rear blades 51 each have a curved shape in which a recessed portion opens in the other direction R2 (the rotational direction of the impeller 35) as viewed in lateral cross section. The stationary blades, or struts, 61 each have a curved shape in which a recessed portion opens in the other direction R2 and in the direction in which the rear blades 51 are located as viewed in lateral cross section.
In the counter-rotating axial flow fan according to the related art, the number N of the front blades 28, the number M of the struts 61, and the number P of the rear blades 51 are each a positive integer, and satisfy a relationship of N>P>M.
Four curved portions 18 and 58 are formed at four corners of both end portions, in the axial direction, of an inner wall portion of the air channel formed by the cylindrical portions 13 and 33. The curved portions 18 and 58 become larger in diameter toward the suction port 15 and the discharge port 57, respectively. The four curved portions 18 and 58 are shaped such that defining the diameter of the inner wall portion of the air channel as Ro, the maximum diameter Rm of the curved portions 18 and 58 is approximately 1.06Ro at ends of the cylindrical portions 13 and 33 where the diameters of the curved portions 18 and 58 are the largest. In addition, defining the outside diameter of the front blades 28 as Rf, the minimum clearance Cf between the front blades 28 and the struts 61 is less than Rf/6. Moreover, defining the outside diameter of the rear blades 51 as Rr, the minimum clearance Cr between the rear blades 51 and the struts 61 is less than Rr/8.
While the counter-rotating axial flow fan according to the related art can improve the air flow-static pressure characteristics, it is further desired to reduce power consumption and noise.